The fate of heavy metals during pyrolysis of oil-based drill cuttings from shale gas exploitation industry

The exploitation of shale gas generates significant hazardous oil-based drill cuttings (OBDC). This study investigates the migration and transformation of heavy metals during OBDC pyrolysis. Results show heavy metals with lower melting points readily entered liquid and gas phases. The mass reductions were Cd (10.56 % to 39.04 %), Cr (17.5 % to 43.2 %), and Ni (12.6 % to 24.9 %), reducing the heavy metal content and ecological risk of the residual char. Except for Pb in acid leaching, the leaching toxicity was markedly reduced in both acid and water leaching. However, the speciation of heavy metals shifted towards more unstable speciation, sharply enhancing the bioavailability of OBDC char after pyrolysis. Analysis of variance (ANOVA) results demonstrate a strong correlation (p-value < 0.05) between catalyst type and the bioavailability of Cd, Ni and Zn. The bioavailability of Cu and Pb showed a low correlation (p-value > 0.1) with pyrolysis temperature, heating rate, or catalyst type, while the bioavailability of Cr was significantly correlated (p-value < 0.05) with the above three factors. This study distinguishes itself by comprehensively linking operational parameters to heavy metal migration and transformation, which have been rarely addressed in previous OBDC pyrolysis studies. The findings emphasize the need for integrated control strategies to mitigate secondary pollution from both volatile heavy metals and bioavailable char in practical applications.